1. Field of the Invention
The present invention relates to electrocardiography, and, more particularly, to a system and method for differentiating cardiac memory T-wave inversion from ischemic inversion.
2. Related Art
T-wave inversion (TWI) has a wide range of etiologies, from a normal variant to hypertrophic cardiomyopathy, pericarditis, and life-threatening myocardial ischemia. The majority of TWI falls in a category of “nonspecific ST-T-wave abnormalities” and accounts for 50% to 70% of abnormal tracings in general hospital populations. Interpretation of these ECGs is based primarily on correlation with available clinical data.
Post-pacing precordial T-wave inversions, known as cardiac memory, mimic anterior myocardial ischemia, and there are no established electrocardiographic criteria that adequately distinguish between the two. This phenomenon is well known to cardiologists. Cardiac memory is usually exhibited when a heart is paced for some period of time, and then the pacing is stopped. The cardiac memory effect usually depends on how long the heart was paced, and can last anywhere from a few hours to many weeks. Frequently, the T-wave following the pacing appears inverted. This is commonly referred to as T-wave inversion, or TWI. A similar TWI effect is frequently observed in ischemic patients. Specifically, post-pacing precordial T-wave inversion mimics anterior myocardial ischemia.
Cardiac memory is one of the benign causes of precordial TWI. ECG patterns of cardiac memory are manifested upon resumption of a sinus rhythm after a period of abnormal ventricular activation, such as ventricular pacing, transient left bundle branch block, ventricular arrhythmias, or WPW (Wolff Parkinson White syndrome). The most common cause of cardiac memory is ventricular pacing. Because T-wave changes of cardiac memory may persist for long periods of time after the pacing is discontinued, their causal relationship is often obscured. Although the benign nature of cardiac memory TWI is well established, no reliable diagnostic mechanisms have been described to differentiate pacing-induced cardiac memory from T-wave inversions resulting from anterior wall ischemia and infarction.
While the cardiac memory-induced T-wave inversion is a generally harmless phenomenon that usually disappears over time, ischemia is a serious problem, normally treated by coronary angioplasty, stenting or coronary bypass surgery. Ischemia is probably the most dangerous cause of T-wave inversion.
Because of the difficulty in distinguishing between the two causes of TWI, as well as in distinguishing causes of TWI in patients with pacemakers, many physicians, upon seeing T-wave inversion, are compelled to perform expensive and unnecessary catheterizations, angiograms, hospital admissions, time-consuming and costly evaluations to rule out ischemia, and other tests that would not be performed had the physician known that the T-wave inversion is due to cardiac memory, and not ischemia. Most physicians, in fact, when they see an inverted T-wave, assume the worst. Similarly, much of the automated diagnostic equipment, upon detection of an inverted T-wave, gives a diagnosis of possible ischemia.
Accordingly, there is a need in the art for a simple method of differentiating between benign cardiac memory-induced T-wave inversion, and ischemia-induced inversion.